Corvette Carburetor Rebuild - Carter AFB Blueprint Rebuild

High-Performance Carburetor From The 327 V-8 Heydays

At the dawn of the muscle car era, GM had thoroughly tested Carter's AFB carburetors on Chevrolet's 348 and 409, and on the high-output Pontiac 389 V-8s. The AFB was a logical choice for Corvette's new 327 small-block V-8s. From 1962-'65, a single, high-flow Carter AFB four-barrel carburetor fueled the 300 and 340 horsepower engines. Until the 1964 introduction of the Holley 4150 carburetor on 365 horsepower L76 engines, only Rochester's mechanical fuel injection could top the AFB's performance.

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Carter's AFB (aluminum four-barrel) made history from 1957 through the 1960s. For high-performance engines throughout the industry, the AFB and its successor, the AVS, were common picks. Topping the muscle car ratings for 1962, Chevrolet's big-block 409, equipped with dual AFBs, produced 409 scorching horsepower. Not outdone, the Corvette squeezed 340 horsepower from a mere 327 cubic inches, using a performance mechanical lifter camshaft, 11.25:1 compression, and a single Carter AFB carburetor!

The needs of an AFB carburetor are minimal. Unlike the finicky Holley square flange designs, the AFB holds a tune well. Less susceptible to altitude and atmospheric effects, the AFB requires less tinkering at trackside. Owners of 300 and 340 horsepower 327s attest to the driving pleasure and tuning ease that the AFB offers.

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Among the desirable features of the AFB is its "dual two-barrel" design. The primary barrels functioned as a true two-barrel carburetor, featuring low-speed, high-speed, accelerating, and power systems. The choke, as customary, operates on the front two barrels. Secondary throttles open mechanically as the throttle linkage reaches a predetermined stage. Vacuum actuated auxiliary valves smooth the transition into the secondary barrels. The mechanically actuated linkage, complemented by counterbalanced and vacuum actuated auxiliary valves, assures predictable and stable performance.

The track tuning of an AFB is not difficult. Vacuum operated step-up pistons have calibrated counterbalance springs. Metering rods attach to these pistons, which can be accessed with the air cleaner removed and without removing the carburetor's air horn. Float adjustment does require removal of the air horn. Once adjusted properly, the two floats hold their settings between overhauls.

Blueprint Rebuilding the Carter AFB
Due to its spare design, the AFB is reliable and comparatively easy to service. A properly tuned AFB provides easy starts, a stable idle, and quality performance-without tinkering. These carburetors, like any other alloy type, are vulnerable to throttle shaft wear and main body throttle bore wallowing. In that regard, however, the AFB holds up better than most.

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One of the most satisfying aspects of AFB work is the refinement of tune. An AFB's adjustments, like other carburetors, are critical and must be within close tolerances. Following that standard, the rebuilding process focuses on the various features and adjustments required. With adequate time spent, a builder can fine-tune an AFB to perform as designed. Once set properly, the AFB provides exceptional driving satisfaction.

Rebuilding always involves careful disassembly and cleaning. Sequence of disassembly is important, as the delicate metering rods and other brass parts require protection. For years, parts like the rods have not been available outside of NOS trackside tuning kits. (These rare and obsolete NOS kits have sold for $1,000 or more!) Avoid damaging parts. A numbers-correct AFB carburetor, in rebuildable condition, is a valuable part of your engine package.

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Rebuilding and tuning were once routine, periodic service measures. During the high-octane, Tetra Ethyl Leaded-fuel era, a Corvette equipped with an AFB would require carburetor rebuilding-or at least a "boil-out"-every 25,000 miles. With a quality rebuilding kit and proper adjustments, a rebuild should last at least that mileage. One deciding factor would be how much the car sets idle, as varnish buildup and jet clogging occur rapidly as fuel decays. Use of a fuel stabilizer would be wise if your Corvette sets up for long intervals.

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Carter's AFB carburetor, introduced to the automotive industry in 1957, became Corvette's choice for the 300 and 340 horsepower RPO 583 and RPO 396 options in 1962. Exceptional stability and predictable performance were AFB hallmarks.

This is a typical high-mileage AFB. Dirt, grime, clogging of jets, and gummy venturi occur over time. The carburetor that sets up for long periods with stale gasoline in its bowls is a candidate for problems.

Varnish buildup accrues. Spillage from worn gaskets creates leaks. Gaskets shrink and loosen over time, which leads to gasoline seepage. Leaded fuel leaves deposits in the bowl floors. This rebuildable carburetor looks typical for its mileage.

Check each throttle shaft for side play. Remove the step-up pistons, springs, and rods. These rods meter fuel under various engine loads and throttle positions. Rods slide up and down within replaceable jets. Jets screw into the main body floor.

Air horn removed, the float bowls and floor of the carburetor look typical for the age and mileage of this carburetor. The accelerator pump has been replaced in an earlier rebuild and looks dehydrated from setting up for a long period.

Air horn flipped over, the two floats and needle assemblies are visible. The needles, seats, and gaskets will be replaced. All metal parts require cleaning. Brass floats hold up well and seldom need replacement unless signs of cracking or leaks exist.

Needle seat has a soft screwdriver slot. When loosening a seasoned needle seat, resistance may distort the slot. Here, a common screw extractor removes the old seat. Be certain to remove any brass debris that sloughs into fuel passageways.

Parts clean up thoroughly with a dip in caustic carburetor cleaner and a water rinse. Any part with passageways should be blown out, using a clean, dry compressed air source. Venturi and body passages require flow testing with compressed air.

Compare jet sizes with those listed in the factory workshop manual or Carter specifications. Passageways cleared, reinstall the jets. Use a screwdriver that fills the slot. Avoid damaging the brass. Tighten jets securely.

Note the dual accelerator pump wells on this carburetor. The single pump goes to the jetted side of the body. The unused pump well is easy to identify, as there is no accelerator pump hole in the air horn.

Install the new needle seats and supplied gaskets. Secure seats with the correct tool. The driver shown fits vintage Chevrolet or Corvette "Blue Flame" six-cylinder distributors and works nicely here. A broad-tipped screwdriver will suffice.

Needle and float in place, you can check the float measurements. Do not press the needle into its seat, as damage to a Viton or brass tip can result. Let the float fall naturally into position. Measure and adjust the floats to factory specification.

An alternative to the straight, 32nds-inch ruler is a caliper. Here, float height matches specs for this AFB application. The air horn gasket in place, floats resting against their needles, read the measurement at the toe of each float.

Float drop is a crucial measurement with the AFB. Too much drop and the needle can stick in the open position. Not enough float drop will restrict fuel flow. Specs call for a new gasket in place, measuring at the toe of the float.

Floats adjusted properly, the air horn can be carefully installed on the main body. Note that all main body parts were installed before placing the air horn on the body. Step-up pistons, springs, and metering rods install from the topside of the air horn.

Polish step-up pistons and rods as necessary. They must move freely in the body of the carburetor. Springs must be in excellent condition, as their precise calibration counters the amount of vacuum pull at each piston.

Accelerator pump rod height is another crucial measurement. Linkage installed and adjusted properly will determine the pump stroke and timing of the accelerator pump. Here, the depth gauge end of a caliper makes this measurement easier.

The choke housing and piston look like new after the dipping process. The choke mechanism must move freely, without excess piston play. Remedy any cold start or warm-up issues here. Make sure the bimetallic spring and cover are in top condition.

New housing gasket, hold-down springs, and screws restore this choke housing. Handle the cover with care; these parts are fragile and becoming rare. Adjust the spring tension and housing position to factory or carburetor kit specifications.

A new fuel inlet filter and gasket will help prevent contamination of your freshly built carburetor. Brass fitting is the original but looks new after polishing with a soft wire wheel. Simple detailing like this pays off in appearance points.

Completely assembled, the carburetor looks new. Bead-blasting restored the body and air horn surfaces, and the original "tag" is now in place. The tag number and casting numbers on the carburetor body determine authenticity.

Choke linkage, secondary linkage, and function of the throttle and choke valves should be checked on the bench. Make sure each adjustment is on specification before installing the carburetor on the engine.

With times getting tough, a brand-new shiny carb may be out of reach for some enthusiasts, but a decent used carb may not and rebuilding it was never easier with Holley's 4150 Carb rebuild kit. - Super Chevy Magazine » Read More